The semiconductor or IC industry aims to manufacture ICs with higher and higher densities of devices on a smaller chip area to achieve greater functionality and to reduce manufacturing costs. This desire for large scale integration has led to a continued shrinking of circuit dimensions and device features. The ability to reduce the size of structures, such as gate lengths in field-effect transistors and the width of conductive lines, is driven by lithographic performance.
With conventional photolithography systems, light is provided through or reflected off a mask or reticle to form an image on a semiconductor wafer. Generally, the image is focused on the wafer to expose and pattern a layer of material, such as photoresist material, that is disposed on a target layer to be processed. In turn, the patterned photoresist material is utilized to define doping regions, deposition regions, etching regions, or other structures associated with ICs in one or more target layers of the semiconductor wafer. The photoresist material can also define conductive lines or conductive pads associated with metal layers of an IC. Further, the photoresist material can define isolation regions, transistor gates, or other transistor structures and elements.
As the number of individual devices incorporated in the design of a semiconductor integrated circuit increases, there is a growing need to decrease the minimum feature size, that is, the minimum width, the minimum space between individual elements of the devices, the minimum widths of holes or vias, and the like. As the minimum feature size decreases, it becomes increasingly difficult to adequately resolve the features during photolithography because of reflection of light from the photoresist material/target layer interface. Optical distortion causes a loss of the anticipated one-to-one correspondence between the image on the mask and the image created in the patterned photoresist material.
Bottom anti-reflective coatings (BARCs) are known and used to mitigate defects during the patterning of the target layer by attenuating or absorbing the light waves reflected from the target layer surface during photo exposure operations to improve image contrast. BARCs are typically interposed between the target layer and the photoresist so as to serve as a barrier that inhibits the reflected waves from traversing back through the photoresist and adversely affecting the imaging process, which helps in defining images. As feature sizes approach 45 nm and less, lithography using ArF exposure systems with high or hyper numerical aperture (NA) in the range of about 1.30 to about 1.35 is typically required so that the incident light rays can be projected at high propagation angles, which improves resolution. However, at such large propagation angles, the reflectance from the photoresist/BARC interface substantially increases.
Several solutions have been suggested to overcome the problems associated with increases in reflectance. For example, the use of two or more BARC films with different indices of refraction (n) and absorbance (k) have been recommended to overcome the shortcomings of single BARC systems. However, the use of two or more BARC films requires the deposition of two different films, both of which should be deposited optimally. This in turn, increases costs, decreases yield and throughput, and can result in higher defectivity. Graded BARCs with optical properties that change as a function of the thickness of the BARC also have been noted. These graded BARCs are fabricated using specialized vapor deposition processes that permit the composition of the BARC to change as the BARC is deposited. However, such process can be costly and require materials, equipment and processes that are not suitable for commercial production. Alternately, graded BARCs may be prepared using spin-on processes in which a graded material is produced by interfacial segregation of components in the spin-on formulation. In these instances, very precise control of materials behavior is required, which may ultimately limit general utility and achievable reflectivity control.
Accordingly, it is desirable to provide photolithography methods that utilize easily-integrated BARCs with graded optical properties during photolithography. In addition, it is desirable to provide methods for performing photolithography that allow for high NA imaging conditions while providing means for controlling reflection in an effective and cost-efficient manner. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.